As is known in the art, certain metal hydride alloy materials are capable of absorbing and desorbing hydrogen. These materials can be used as hydrogen storage media and/or as electrode materials for fuel cells, and metal hydride batteries including metal hydride/air battery systems.
When an electrical potential is applied between the cathode and a metal hydride anode in a metal hydride cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion; upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron. The reactions that take place at the positive electrode of a nickel metal hydride cell are also reversible. Most metal hydride cells use a nickel hydroxide positive electrode. The following charge and discharge reactions take place at a nickel hydroxide positive electrode.

In a metal hydride cell having a nickel hydroxide positive electrode and a hydrogen storage negative electrode, the electrodes are typically separated by a non-woven, felted, nylon or polypropylene separator. The electrolyte is usually an alkaline aqueous electrolyte, for example, 20 to 45 weight percent potassium hydroxide.
One particular group of metal hydride materials having utility in metal hydride battery systems is known as the ABx class of material with reference to the crystalline sites that its member component elements occupy. ABx type materials are disclosed, for example, in U.S. Pat. No. 5,536,591 and U.S. Pat. No. 6,210,498, the disclosures of which are incorporated herein by reference. Such materials may include, but are not limited to, modified LaNi5 type as well as the TiVZrNi type active materials. These materials reversibly form hydrides in order to store hydrogen. Such materials utilize a generic Ti—V—Ni composition, where at least Ti, V, and Ni are present with at least one or more of Cr, Zr, and Al. The materials are multiphase materials, which may contain, but are not limited to, one or more TiVZrNi type phases with a C14 and C15 type crystal structure. Some specific formulations comprise:(TiV2-xNix)1-yMy where x is between 0.2 and 1.0; y is between 0.0 and 0.2; and M=Al or Zr;Ti2-xZrxV4-yNiy where Zr is partially substituted for Ti; x is between 0.0 and 1.5; and y is between 0.6 and 3.5; andTi1-xCrxV2-yNiy where Cr is partially substituted for Ti; x is between 0.0 and 0.75; and y is between 0.2 and 1.0.
Other Ti—V—Zr—Ni alloys may also be used for a rechargeable hydrogen storage negative electrode. One such family of materials is a specific sub-class of these Ti—V—Ni—Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. In a particular instance, the alloy has the composition(Ti2-xZrxV4-yNiy)1-zCrz where x is from 0.00 to 1.5, y is from 0.6 to 3.5, and z is an effective amount less than 0.20. These alloys may be viewed stoichiometrically as comprising 80 atomic percent of a V—Ti—Zr—Ni moiety and up to 20 atomic percent Cr, where the ratio of (Ti+Zr+Cr+ optional modifiers) to (Ni+V+ optional modifiers) is between 0.40 to 0.67. These alloys may include additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components.
The V—Ti—Zr—Ni family of alloys has an inherently higher discharge rate capability than previously described alloys. This is the result of substantially higher surface areas at the metal/electrolyte interface for electrodes made from the V—Ti—Zr—Ni materials. The surface roughness factor (total surface area divided by geometric surface area) of V—Ti—Zr—Ni alloys is about 10,000. This value indicates a very high surface area and is supported by the inherently high rate capability of these materials. The characteristic surface roughness of the metal/electrolyte interface is a result of the disordered nature of the material. Since all of the constituent elements, as well as many alloys and phases of them, are present throughout the metal, they are also represented at the surfaces and at cracks which form in the metal/electrolyte interface. Thus, the characteristic surface roughness is descriptive of the interaction of the physical and chemical properties of the host metals as well as of the alloys and crystallographic phases of the alloys in an alkaline environment. These microscopic chemical, physical, and crystallographic parameters of the individual phases within the hydrogen storage alloy material are believed to be important in determining its macroscopic electrochemical characteristics.
In addition to the physical nature of its roughened surface, it has been observed that V—Ti—Zr—Ni alloys tend to reach a steady state surface composition and particle size. This steady state surface composition is characterized by a relatively high concentration of metallic nickel. These observations are consistent with a relatively high rate of removal through precipitation of the oxides of titanium and zirconium from the surface and a much lower rate of nickel solubilization, providing a degree of porosity to the surface. The resultant surface seems to have a higher concentration of nickel than would be expected from the bulk composition of the negative hydrogen storage electrode. Nickel in the metallic state is electrically conductive and catalytic, imparting these properties to the surface. As a result, the surface of the negative hydrogen storage electrode is more catalytic and conductive than if the surface contained a higher concentration of insulating oxides.
In contrast to the Ti—V—Zr—Ni based alloys described above, alloys of the modified LaNi5 type have generally been considered “ordered” materials that have a different chemistry and microstructure, and exhibit different electrochemical characteristics compared to the Ti—V—Zr—Ni alloys. However, analysis reveals while the early unmodified LaNi5 type alloys may have been ordered materials, the more recently developed, highly modified LaNi5 alloys are not. The performance of the early ordered LaNi5 materials was poor. However, the modified LaNi5 alloys presently in use have a high degree of modification (that is as the number and amount of elemental modifiers has increased) and the performance of these alloys has improved significantly. This is due to the disorder contributed by the modifiers as well as their electrical and chemical properties.
U.S. Pat. No. 5,536,591 considers the compositional microstructure of hydrogen storage alloys in greater detail and recognizes that the composition of hydrogen storage alloys is more complicated than is indicated by the nominal or bulk composition. Specifically, the '591 patent recognizes the importance of a surface oxide layer that is typically present in hydrogen storage alloys, and its influence on the charging and discharging processes. In electrochemically driven processes, for example, the oxide layer constitutes an interface between the electrolyte and the bulk hydrogen storage alloy and accordingly may also be referred to as an interface layer or region. Since oxide layers are typically insulating, they generally inhibit the performance of electrodes utilizing metals or metal alloys. Prior to electrochemical reaction, metal or metal alloy electrodes are typically activated, a process in which the surface oxide layer is removed, reduced or modified to improve performance. The process of activation may be accomplished, for example, by etching, electrical forming, pre-conditioning or other methods suitable for removing or altering excess oxides or hydroxides. See, for example, U.S. Pat. No. 4,717,088, the disclosure of which is hereby incorporated by reference.
The '591 patent extended the Ovshinsky principles to the oxide layer of hydrogen storage materials and thereby demonstrated improved catalytic activity. Specifically, hydrogen storage alloys having Ni-enriched catalytic regions in the oxide layer are shown to have high catalytic activity. The Ni-enriched catalytic regions may be prepared, for example, through an activation process in which elements of the hydrogen storage alloy other than Ni are preferentially corroded to provide regions of metallic nickel alloy of about 50-70 angstroms distributed throughout the oxide layer. The Ni-enriched catalytic regions function as catalytic sites having high activity. Formation of the Ni-enriched catalytic regions of the '591 patent is promoted by a pre-activation thermal annealing step. The annealing step acts to condition the surface region of a hydrogen storage alloy and renders it more susceptible to the formation of Ni-enriched catalytic regions during activation.
U.S. Pat. No. 4,716,088, the disclosure of which is incorporated herein by reference, discloses, inter alia, a process for activating metal hydride storage materials to alter the relatively thin, but very dense surface oxide interface layer separating the bulk alloy material forming the negative electrode in a nickel metal hydride battery from the electrolyte (such as KOH). In the activation process, the thin surface oxide thickens as it is further oxidized upon exposure to the electrolyte. However, the oxide also becomes more porous and thereby allows electrolyte to interact with the bulk metal and provide a pathway for the chemical reactions, specifically shuttling of hydrogen ions from the bulk metal alloy to the electrolyte.
Improving drastically on the disclosure of the '088 patent, the '591 patent drastically changes the thicker, porous surface oxide formed by the activation process taught by the '088 patent. The inventors thereof surprisingly discovered that the steady state surface oxide of the '088 patent could be characterized as having a relatively high concentration of metallic nickel. An aspect of the '591 patent is that, by subjecting the metal hydride alloy to a relative lengthy soak in KOH solution, at elevated temperature, a significant increase in the frequency of occurrence of these nickel regions as well as a more pronounced localization of these regions. More specifically, the materials of the '591 patent have enriched nickel regions of 50-70 angstroms in diameter distributed throughout the oxide interface and varying in proximity from 2-300 angstroms, preferably 50-100 angstroms, from region to region. As a result of the increase in the frequency of occurrence of these nickel regions, the materials of the '591 patent exhibit increased catalysis and conductivity.
The increased density of Ni regions in the '591 patent provides powder particles having an enriched Ni surface. Prior to the '591 patent, Ni enrichment was attempted unsuccessfully using microencapsulation. The method of Ni microencapsulation results in the deposition of a layer of Ni about 100 angstroms thick at the metal-electrolyte interface. Such an amount is excessive and results in no improvement of performance characteristics.
The enriched Ni regions of the '591 patent can be formed via the following fabrication strategy: Specifically formulate an alloy having a surface region that is preferentially corroded during activation to produce the enriched Ni regions. As stated in the '591 patent, it is believed that Ni is in association with an element such as Al at specific surface regions and that this element corrodes preferentially during activation, leaving the enriched Ni regions of the '591 patent. “Activation” as used herein and in the '591 patent refers to “etching” or other methods of removing excessive oxides, such as described in the '088 patent, as applied to electrode alloy powder, the finished electrode, or at any point in between in order to improve the hydrogen transfer rate.
The Ni-enriched catalytic regions of the '591 patent are discrete regions. The catalytic activity of the Ni-enriched catalytic regions is controllable by controlling their size, separation, chemical composition and local topology. In one embodiment of the '591 patent, the discrete Ni-enriched catalytic regions include metallic Ni particles having a diameter of 50-70 angstroms or less that are separated from each other by distances of 2-300 angstroms. The Ni-enriched catalytic regions are distributed throughout the oxide layer. The portions of the oxide layer surrounding the Ni-enriched catalytic regions or catalytic metallic Ni particles are referred to as the support matrix, supporting matrix, supporting oxide, oxide support or the like. The Ni-enriched catalytic regions are thus supported by or within the support matrix. The support matrix may include fine and coarse grained oxides and/or hydroxides of one or more of the metallic elements present in the hydrogen storage alloy composition and may also include multiple phases, some of which may be microcrystalline, nanocrystalline or amorphous.
Further improvements over the alloys of the '591 patent are disclosed in U.S. Pat. No. 6,740,448, the disclosure of which is incorporated herein by reference, wherein it is taught that superior catalysis and high rate discharge performance can be achieved by one or more of the following: 1) the catalytic metallic sites of the alloys are formed from a nickel alloy such as NiMnCoTi rather than just Ni; 2) the catalytic metallic sites of the alloys are converted by elemental substitution to an FCC structure from the BCC structure of the prior art Ni sites; 3) the catalytic metallic sites of the alloys are much smaller in size (10-50, preferably 10-40, most preferably 10-30 angstroms) than the Ni sites of the prior art alloys (50-70 angstroms) and have a finer distribution (closer proximity); 4) the catalytic metallic sites of the alloys are surrounded by an oxide of a multivalent material (containing MnOx) which is believed to possibly be catalytic as well, as opposed to the ZrTi oxide which surrounded the prior art Ni sites; 5) the oxide could also be multiphase with very small (10-20 angstroms) Ni particles finely distributed in a MnCoTi oxide matrix; 6) the oxide may be a mix of fine and coarse grained oxides with finely dispersed catalytic metallic sites; 7) alloy modification with aluminum may suppress nucleation of large (50-70 angstroms) catalytic metallic sites (at 100 angstrom proximity) into a more desirable “catalytic cloud” (10-20 angstroms in size and 10-20 angstroms proximity); 8) NiMn oxide is the predominant microcrystalline phase in the oxide and the catalytic metallic sites may be coated with NiMn oxide.
The oxide surface of the alloys of the '448 patent is the same thickness as that of the prior art alloys; however, the modification of those alloys is described as affecting the oxide surface in several beneficial ways. First the oxide accessibility has been affected. That is, the additives to the alloy have increased the porosity and the surface area of the oxide. This is suggested to be caused by Al, Sn and Co. The modifiers added to the alloy are readily soluble in the electrolyte and believed to “dissolve” out of the surface of the alloy material, leaving a less dense, more porous surface into which the electrolyte and ions can easily diffuse. Second, the inventors of the '448 patent have noted that the derivative alloys have a higher surface area than the prior art alloys, and it is believed that the mechanical properties of the alloy (i.e. hardness, ductility, etc.) have been affected. This allows the material to be crushed easier, and allows for more microcracks to be formed in the alloy material during production and also easier in-situ formation of microcracks during electrochemical formation. Finally, the inventors of the '448 patent have noted that the alloys are more catalytically active than the prior art alloys. This is believed to be caused by a more catalytic active oxide surface layer. This surface layer, as is the case with some prior art materials (see for example U.S. Pat. No. 5,536,591 to Fetcenko et al.), includes nickel particles therein. These nickel particles are believed to provide the alloy with its surface catalytic activity. In the alloy of the '448 patent, the inventors believe there are a number of factors causing the instant increase in catalytic surface activity. First, the inventors believe that the nickel particles are smaller and more evenly dispersed in the oxide surface of the instant alloy materials. The nickel particles are believed to be on the order of 10 to 50 angstroms in size. Second, the inventors believe that the nickel particles may also include other elements such as cobalt, manganese and iron. These additional elements may enhance the catalytic activity of the nickel particles, possibly by increasing the roughness and surface area of the nickel catalytic sites themselves. Third, the inventors of the '448 patent believe that the oxide layer itself is microcrystalline and has smaller crystallites than prior art oxide. This is believed to increase catalytic activity by providing grain boundaries within the oxide itself along which ions, such as hydrogen and hydroxyl ions, may move more freely to the nickel catalyst particles which are situated in the grain boundaries. Finally, the instant inventors have noted that the concentrations of cobalt, manganese and iron in the oxide surface are higher than in the bulk alloy and higher than expected in the oxide layer.
The surface area of the alloy of the '448 patent increases in surface area by about a factor of four during treatment, and the higher surface area of the alloy is only partially responsible for the higher catalytic property of these alloys. As the AC impedance measurements demonstrated, the better catalytic activity of the surface of the inventive alloy also contributes to the enhanced catalytic behavior thereof.
Hence, the improved power and rate capability of the alloys of the '448 patent is suggested to be the result of the higher surface area within the surface oxide as well as improved catalytic activity within the oxide due to the smaller size and finer dispersion of the nickel catalyst particles compared to prior art materials. Observations from high resolution scanning transmission electron microscopy (STEM) included presence of nickel catalyst “clouds” having a size in the 10-30 angstrom range and extremely close proximity, on the order of 10-20 and 10-50 angstrom distance. Another contributing factor to the improved catalysis shown by the alloys of the '448 patent is the transformation of the supporting oxide in which the Ni particles reside.
In other prior art materials, the supporting oxide may be primarily rare earth or TiZr based oxides while in the case of the materials of the '448 patent, the support oxide is now comprised of at least regions of NiCoMnTi “super catalysts.” This could also be NiMn regions surrounded by TiZr oxide. These super catalysts show a surprising lack of oxygen based on Electron Energy Loss Spectroscopy (EELS). It may be possible these regions are partially metallic or in a low oxidation state.
Another observation with the materials of the '448 patent is that prior art nickel catalytic regions within the oxide were BCC crystallographic orientation based on Select Area Electron Diffraction (SAED), which the inventive materials were observed to have an FCC orientation. It may be possible that the catalytic regions of Ni have been partially substituted by Co, Al, Mn, Sn, or other elements which have shifted the crystallographic orientation. It is indeed likely the BCC to FCC Ni shift reflects a higher degree of substitution. The inventors of the '448 patent theorize that it is also possible the FCC Ni in conjunction with NiCoMnTi regions and TiZr oxide may form a super lattice which may further promote ionic diffusion and reaction. Still another theory based on analytical evidence suggests that metallic Ni particles reside in a Mn oxide support. The presence of the Mn oxide is intriguing in that MnOx is multivalent and could promote catalysis via changing oxide states during the charge/discharge reactions.
Finally, another interpretation of the analytical evidence of the '448 patent suggests even a multiphase surface oxide. In addition to metallic Ni or Ni alloys, there appears to exist both a fine grained and coarse grained support oxide. It is suggested that the coarse grained aspect to the surface is dominated by TiZr prior art style oxide while the appearance of the fine grained support oxide in the materials may be the MnOx or NiMnCoTi oxide or a MnCoTi oxide.
The supporting matrix and catalytic sites thereof are further discussed in U.S. Pat. No. 6,270,719 (the '719 patent) to Fetcenko, Ovshinsky, and colleagues. The '719 patent teaches additional modification of Ni-enriched regions to provide further improvements in catalytic activity. The '719 patent teaches formation of catalytically active metal-enriched regions comprising not only metallic Ni particles, but also particles of metal alloys such as alloys of Ni with one or more of Co, Cr, V, Pt, Pd, Au, Ag, Rh, Ti, Mn, or Al as well as other metal alloys (e.g. PtAu). The '719 patent further teaches that alloying may provide particles having an FCC structure instead of the BCC structure of the metallic Ni particles of the '591 patent.
The instant invention further considers the nature of the oxide support layer of hydrogen storage alloys and is particularly concerned with extending the Ovshinsky principles to the microstructure of the support matrix in order to obtain improved performance of electrochemical and thermal hydrogen storage alloys. The performance of hydrogen storage materials is based on factors that include the intrinsic activity of catalytic sites, the number of catalytic sites, interactions between catalytic sites, interactions between catalytic sites and hydrogen storage sites, the number of hydrogen storage sites and the stability of hydrogen storage sites. These factors influence the hydrogen storage capacity, thermodynamic properties, and kinetics of hydrogen storage materials. The prior patents described hereinabove have demonstrated various ways to improve the activity of catalytic sites, the number of catalytic sites, the number of hydrogen storage sites, and the stability of hydrogen storage sites.
U.S. Pat. No. 6,830,725, the disclosure of which is incorporated herein by reference, discusses additional features of the support matrix and/or catalytic metallic regions or particles that are beneficial to the performance of hydrogen storage materials. More specifically, the '725 patent is concerned with beneficial modifications of the region at or near the surface of a hydrogen storage alloy. The region at or near the surface of a hydrogen storage alloy may also be referred to herein as the surface or interface region, surface or interface layer, surface or interface oxide or the like. The surface or interface region constitutes an interface between the electrolyte and the bulk portion of an electrochemical hydrogen storage alloy. In one embodiment of the '725 patent, the interface region includes catalytic metal or metal alloy particles having angstrom scale dimensions that are supported by a surrounding support matrix having a higher degree of porosity than with previously known metal hydride alloys. As described therein, the relative proportions of catalytic metal or metal alloy particles and support matrix in the surface region vary with the composition and processing treatments of the instant hydrogen storage alloys.
The '725 patent describes a process for tuning the microstructure of the support matrix in the interface region of hydrogen storage alloys so as to create a more open network structure that facilitates the access of reactant species to catalytic sites and the departure of product species away from catalytic sites through voids or channels in the interface region. Voids and channels of sufficient size relative to participating reactant species (in charging or discharging processes) facilitate the mobility of reactant species and may be referred to as reactant voids or channels. The presence of reactant voids or channels in the interface region of the instant alloys can lead to greater utilization of catalytic sites and improved performance, particularly at low temperature. Another aspect of the '725 patent focuses on tuning the microstructure of the interface region of hydrogen storage alloys so as to increase the density of catalytic sites. A greater number of catalytic sites in a given volume of hydrogen storage alloy leads to an increase in overall catalytic reactivity.
As will be explained in detail hereinbelow, the present invention incorporates and builds on the above-described techniques and, among other things, builds on the teaching of the prior art so as to further improve the surface morphology, and hence the three-dimensional configuration and the catalytic activity of the hydrogen storage alloy materials in general, and their surface interface regions in particular. However, the improvement taught by the instant inventors is not trivial. The analysis performed on the subject microstructurally tuned interface surface reveals that for the first time the interfacial surface layer is not the same throughout. A particle of hydrogen storage alloy material has a huge surface area and therefore a huge amount of interfacial surface exposed to the electrolyte. Heretofore, analyses of the various areas of the surface oxide revealed identical surface morphologies, i.e., approximately the same density of metallic nickel alloy particles and voids or pores or channels into the surface oxide. For the first time, applicants have changed the morphology of adjacent regions of the interfacial surface. The change in average size of the channels enhances the performance of the alloy, in particular under low temperature conditions. The alloys of the present invention may include modifiers which may hereinafter be referred to as modifying elements, microstructure tuning elements, microstructure modifiers, support matrix modifiers, supporting oxide modifiers, surface or interface region modifiers or the like. The presence of the formula modifiers in combination with other elements provides an overall alloy formulation that provides the beneficial microstructural and porosity effects of the instant invention.
In the absence of microstructure tuning according to the instant invention, the base alloys may have metal enriched catalytic regions that include catalytically active particles comprised of nickel, nickel alloy as well as other metals or metal alloys as described in the '591, '725 and '719 patents.
Microstructure tuning according to the instant invention permits control of the morphology, and in particular the three-dimensional structure, of the interface layer surrounding the catalytically active particles and thereby enhances the mobility of relevant molecules or molecular species in electrochemical or thermal charging or discharging processes with respect to the alloy material. The microstructure of the instant alloys has specifically configured voids or channels which define a three-dimensional structure that facilitates access of reactant species within the surface region as well as to and from catalytic particles or regions. The instant voids or channels include a higher density of catalytic metallic particles therein.
The characteristics and range of modifications of the support matrix surrounding the catalytic metal-enriched regions of the hydrogen storage materials of the prior art have not been fully optimized. Incidental variations of the support matrix as a result of effects intended to improve the performance or number of catalytic and hydrogen storage sites have been mentioned, but no teaching of the intentional modification of the three-dimensional morphology of the support matrix has been presented. In the '591 patent, for example, formation of Ni-enriched regions was believed to provide a somewhat more porous supporting oxide. In the '719 patent, as another example, inclusion of Mn in the bulk composition of the hydrogen storage alloy was proposed to provide a multivalent MnOx component to the oxide layer where the multivalent component may have catalytic properties.
Tuning of the three-dimensional structure and catalytic sites of the channels in the oxide interface layer of the materials of the present invention provides an additional degree of freedom for optimizing the performance of electrochemical and thermal hydrogen storage materials. In addition to the intrinsic activity, number, and interactions among and between catalytic sites, hydrogen storage sites and surrounding material described hereinabove, high performance further requires that a hydrogen bearing source such as hydrogen gas or water has accessibility to a catalytic site. The concept of accessibility further extends to the ability of byproducts formed during charging or products formed during discharging to depart catalytic sites so that the site may be further utilized.
As an example, an electrochemical hydrogen storage alloy that includes metal enriched catalytic regions may be considered wherein the alloy is included as the negative electrode of a rechargeable battery in the presence of an aqueous electrolyte. Upon charging, water accesses a metal enriched catalytic site to form atomic hydrogen for storage and a hydroxyl ion byproduct. In order for this charging process to occur, the support matrix surrounding metal enriched catalytic sites must be sufficiently open or porous to permit water molecules from the electrolyte to access the metal enriched catalytic sites. Additionally, in order to continually effect catalysis at a metal enriched catalytic site, the support matrix must permit hydroxyl ion formed during charging to migrate, diffuse or otherwise depart from the catalytic site so that the access of further water molecules to the catalytic site is not impeded or otherwise blocked by the presence of a hydroxyl ion. Similar considerations apply on discharging. Upon discharging, stored hydrogen combines with hydroxyl ions at a catalytic site to form water. In order to achieve high discharge rates, it is preferable for the support matrix to be sufficiently porous to allow for the facile departure of water molecules formed upon discharging away from the catalytic site. If the departure of water molecules is inhibited by the support matrix, the catalytic site is effectively blocked and additional discharging may be inhibited. Optimal discharging requires not only rapid formation of product, but also rapid departure or transport of products (and byproducts, if present) away from the catalytic site so that the site is available for further participation in the discharge reaction. In addition to reactants, products and byproducts, the accessibility and mobility of ions in the electrolyte to catalytic sites, hydrogen storage sites and within a hydrogen storage material may also be relevant to the overall performance and efficiency of charging and discharging reactions.
Insufficient porosity and/or an inadequate pore morphology of the support matrix may inhibit access to or departure from catalytic sites, for example, by presenting a structure having openings or channels that are too small to provide facile migration of molecular species to and/or from a catalytic site. Thus, even if a particular catalytic site (e.g. within a metal enriched catalytic region or catalytic metallic particle) has high activity, fast kinetics for charging and discharging etc., inability of reactant molecules or electrolyte species to access the catalytic site or inability of product molecules or electrolyte species to depart the catalytic sites may have a deleterious effect on the performance of a hydrogen storage material.
In addition to structural barriers associated with accessing or departing a catalytic site, a supporting matrix may also present steric, electronic or other barriers. Electronic barriers generally arise from intermolecular forces of attraction or repulsion that may be present between the support matrix and migrating or diffusing molecules or chemical species. Electrostatic, van der Waals, bonding, etc. interactions may act to impede migration or diffusion even if sufficiently large structural pathways for migration are available within the support matrix. The concept of porosity as used herein is intended to broadly encompass barriers or inhibitions, regardless of origin, provided by the support matrix to the migration or diffusion of species participating in charging or discharging processes. A highly porous support matrix provides few barriers to migration or diffusion, while a low porosity or highly dense support matrix provides substantial barriers to migration or diffusion.
The ability of a molecule or other chemical species to access or depart a catalytic site may also be referred to as the mobility of the molecule within or with respect to the support matrix. A molecule or chemical species having high mobility is readily able to penetrate, migrate through, diffuse within or otherwise transport through or within the support matrix. High mobility implies greater accessibility of reactants to catalytic sites during charging and greater ability of products to depart from a catalytic site during discharging. High mobility also implies a greater ability of byproducts to depart from a catalytic site during either or both of charging and discharging. High mobility of a species through a support matrix implies that the support matrix provides few barriers (structurally, sterically, electronically, etc.) to migration or diffusion. The transport of electrolyte species is similarly facilitated through a support matrix that provides high mobility. Phenomenologically, species mobility and accessibility to catalytic sites may be manifested in the charge transfer resistance, particularly at low temperature, of an electrochemically driven process. Charge transfer resistance is a measure of the facility of the basic electrodic electron transfer process of an electrochemical reaction. A high charge transfer resistance implies an inhibited electron transfer process. Factors contributing to an inhibition include low number of catalytic sites, low activity of catalytic sites, or inability of relevant molecules and molecular species to access or depart catalytic sites. A highly dense oxide support matrix inhibits the charge transfer process by impeding access to and/or departure from a catalytic site. This inhibition contributes to a large charge transfer resistance and slows the kinetics of an electrochemical process. As the porosity and three-dimensional morphology of the material increases, the charge transfer resistance decreases as species mobility and accessibility to catalytic sites improves. As porosity and morphology are optimized, the support matrix is no longer the dominating factor in determining the charge transfer resistance. Instead, the number and/or activity of catalytic sites or the concentration of reactive species may become controlling.
The mobility of a molecule or other molecular species with respect to a support matrix may be influenced by external factors such as the temperature. Temperature is a relevant consideration because it controls the thermal energy of a molecule. Higher temperatures provide higher thermal energies to molecules and molecular species that access or depart from a catalytic site thereby better enabling them to overcome structural, steric, electronic or other barriers to mobility created by a support matrix. A support matrix that provides sufficient mobility at one temperature with respect to a particular charging or discharging process may not provide sufficient mobility at a lower temperature because of a reduction of thermal energy available to one or more molecules or molecular species requiring access to or departure from a catalytic region. The thermal energy of mobile molecules or species relative to the activation energies of barriers to mobility provided by the support matrix influences the effectiveness of charging and discharging.
The instant invention provides hydrogen storage materials having a preferred three-dimensional support matrix micro and macrostructure and a catalytic ability that enhances the mobility of relevant molecules and molecular species. Mobility enhancements are provided at elevated temperatures, room temperature and low temperatures. Mobility enhancements are provided by the inclusion or formation of specifically configured, catalytically active channels in the surface region of the alloy. In a preferred embodiment, an instant hydrogen storage material is utilized as the active material in the negative electrode of a nickel metal hydride battery that provides superior discharge kinetics at temperatures below 0° C.
In addition to porosity modifications, accelerated and directed preferential corrosion may also lead to a relative local enhancement, at or in the vicinity of the surface, of the concentration of one or more elements that are less susceptible to corrosion. As in the patents incorporated by reference hereinabove, local enhancements in the concentrations of one or more metals may facilitate the formation of metal enriched regions that include catalytic metallic particles.
While not wishing to be bound by theory, the instant inventors believe that the improved morphology of the channel structure of the interface layer and/or increased density and/or optimized size of catalytic metallic particles afforded by the instant invention may, at least in some embodiments of the instant hydrogen storage alloys, occur synergistically. That is, an increase in the porosity and three-dimensional structure of the support matrix may promote the formation of catalytic metallic particles and vice versa. Rather than merely providing local metal enriched regions that include catalytic particles supported on an oxide matrix as in the prior art, the instant invention provides a support matrix comprising a series of convoluted, interconnected voids or channels defining a three-dimensional, sponge-like morphology. In addition, at least portions of the interior surfaces of these interconnected channels are catalytically active and as such include a number of catalytic metallic particles therein.
A key operative feature of the present invention is to provide access between the voids and the catalysts. It is also possible that the introduction of one or more non-modifier elements and/or implementation of one or more chemical processes may also operate to provide the beneficial three-dimensional structural and porosity effects of the instant invention. Such elements and processes can include chemical pretreatments designed to selectively attack one or more of the support oxide elements. For example, HF may provide the final desired oxide porosity. The reader must understand that the subject invention defines, in numerous ways, over the invention disclosed by the assignee in the '725 patent, the disclosure of which applicant considers the closest prior art. First, the increased porosity is due to not only a change in the cross-sectional size of the channels, but also to the three-dimensional shape of those channels as they extend through the surface oxide. While applicant has provided analysis describing channel size, it is to be understood that the size of the openings will vary based on alloy formulations and processing conditions such as preferential corrosion concentrations, duration and temperature. In other words, applicants have supplied additional micro and macrostructural tuning tools that those of ordinary skill in the art may use. Second, the large, three-dimensional channels have the catalytic, metallic nickel alloy particles distributed therethroughout, and a structure of this type is not shown, taught, or obvious from a review of the '725 patent. Third, additional modifiers present in the bulk alloy may now be found in the metallic nickel alloy particles. These are not trivial differences; applicants themselves were surprised to learn of the existence thereof when conducting TEM analysis to understand the reason for the improved electrochemical results they had seen. The electrochemical results due to the vastly improved micro and macrostructure and catalytic activity of the materials of the subject invention move NiMH batteries into the forefront of battery technology with a huge operational temperature range due in part to the large, three-dimensional channels and the ability to accept and deliver huge current densities due to the improved catalysis of the nickel alloy particles which cover the exterior and interior of the surface oxide.
Hydrogen storage materials suitable for microstructure tuning according to the instant invention include base hydrogen storage alloys comprising one or more transition metals or rare earths as well as base alloys in combination with a microstructure tuning element. Base alloys having the formula types AB, AB2, AB5, or A2B and mixtures thereof are within the scope of the instant invention where components A and B may be transition metals, rare earths or combinations thereof in which component A generally has a stronger tendency to form hydrides than component B.
In the base AB hydrogen storage compositions, A is preferably Ti, Zr, V or mixtures or alloys thereof and B is preferably selected from the group consisting of Ni, V, Cr, Co, Mn, Mo, Nb, Al, Mg, Ag, Zn or Pd and mixtures or alloys thereof. Base AB compositions include ZrNi, ZrCo, TiNi, and TiCo as well as modified forms thereof. Representative base AB compositions and modified forms thereof within the scope of the instant invention include those described in U.S. Pat. Nos. 4,623,597; 5,840,440; 5,536,591; and 6,270,719 incorporated by reference hereinabove as well as in U.S. Pat. No. 5,096,667, the disclosure of which is hereby incorporated by reference.
Base A2B compositions include Mg2Ni as well as modified forms thereof according to the Ovshinsky principles in which either or both of Mg and Ni is wholly or partially replaced by a multi-orbital modifier.
Base AB2 compositions are Laves phase compounds and include compositions in which A is Zr, Ti or mixtures or alloys thereof and B is Ni, V, Cr, Mn, Co, Mo, Ta, Nb or mixtures or alloys thereof. The instant invention also includes base AB2 compositions modified according to the Ovshinsky principles described hereinabove. Representative base AB2 compositions within the scope of the instant invention are discussed in U.S. Pat. No. 5,096,667 incorporated by reference hereinabove.
Base AB5 compositions include those in which A is a lanthanide element or a mixture or alloy thereof and B is a transition metal element or a mixture or alloy thereof. LaNi5 is the prototypical base AB5 compound and has been modified in various ways to improve its properties. Ni may be partially replaced by elements including Mn, Co, Al, Cr, Ag, Pd, Rh, Sb, V, or Pt, including combinations thereof. La may be partially replaced by elements including Cc, Pr, Nd, or other rare earths including combinations thereof. Mischmetal may also wholly or partially replace La. The instant invention also includes base AB5 compositions modified according to the Ovshinsky principles described hereinabove. Representative base AB5 compositions within the scope of the instant invention have been discussed in U.S. Pat. Nos. 5,096,667 and 5,536,591 incorporated by reference hereinabove.
Modified Mg-based alloys such as those described in U.S. Pat. Nos. 5,616,432 and 6,193,929, the disclosures of which are hereby incorporated by reference, are also within the scope of the instant invention.
The base alloys of the instant invention may also comprise non-stoichiometric compositions achieved through application of the Ovshinsky principles. Non-stoichiometric compositions are compositions in which the ratio of elements may not be expressible in terms of simple ratios of small whole numbers. Representative non-stoichiometric compositions include AB1±x, AB2±x, AB5±x, and A2B1±x, where x is a measure of the non-stoichiometric compositional deviation. The base alloys of the instant invention may also comprise multiphase materials where a multiphase material is a combination or mixture of materials having different stoichiometries, microstructures and/or structural phases. Structural phases include crystalline phases, microcrystalline phases, nanocrystalline phases and amorphous phases.
In some embodiments, increased support matrix porosity and/or increased density of catalytic metallic particles results from inclusion of a modifying element in the base alloy composition. In other embodiments, inclusion of a modifying element in combination with a reduction in the amount of one or more elements of the base alloy composition provides increased porosity of the support matrix and/or increased density of catalytic metallic particles. In still other embodiments, microstructure tuning occurs through formation, processing, treatment, activation or operation steps as described hereinabove.
The instant hydrogen storage alloys may be prepared by a variety of methods that include melt casting, induction melting, rapid solidification, mechanical alloying, sputtering and gas atomization. An important aspect of the preparation process of many hydrogen storage alloys is a post-formation annealing step in which the material as formed during preparation is subjected to an annealing treatment. The annealing treatment includes heating the material to an elevated temperature for a sufficient period of time. An effect of annealing is to alter or condition the surface of the hydrogen storage material in such a way that the material is susceptible to or responsive to the accelerated and directed preferential corrosion process described hereinabove that leads to formation of angstrom scale catalytic metal or metal alloy particles and greater void volume fraction of, and improved three-dimensional morphology in the surface region. The extent to which accelerated and directed preferential corrosion forms angstrom scale catalytic particles during activation is influenced by the local composition at or near the surface. In the materials of the '591 and '719 patents incorporated by reference hereinabove, local nickel enrichment in the surface region was observed to enable or facilitate formation of angstrom scale catalytic nickel or nickel alloy particles upon activation. A suitable annealing step induces a condition in the surface region in which the nickel concentration is enriched relative to the statistical concentration expected from the formula unit of the hydrogen storage alloy. Annealing under appropriate conditions initiates a segregation of nickel away from the bulk and toward the surface region to provide a nickel enriched surface region.
While not wishing to be bound by theory, the instant inventors believe that formation of a surface region having a sufficiently high nickel concentration enables formation of angstrom scale catalytic nickel or nickel alloy particles upon activation. In addition to a high nickel concentration, a nickel enriched surface region may also include microstructural features that facilitate formation of angstrom scale catalytic nickel or nickel alloy particles. The annealing induced segregation, for example, may be accompanied by local changes in phase, structure, crystallinity, grains, interfaces, etc. in the surface region that may be conducive to formation of angstrom scale catalytic nickel or nickel alloy particles during activation. In connection with the materials of the '591 patent, the instant inventors have demonstrated that angstrom scale catalytic nickel or nickel alloy particles do not form upon activation of materials that have not been subjected to an annealing step. Instead of unoxidized metallic nickel or nickel alloy particles, the surface region of unannealed materials comprises oxidized nickel in the form of an Nin+-rich oxide phase.
The segregation effect observed upon annealing the materials of the '591 patent is believed to be enhanced under the influence of microstructure tuning as described for example in the '725 patent. Inclusion of a microstructure tuning element, for example, may lead to greater segregation of nickel and a greater local enrichment of nickel concentration in the instant hydrogen storage alloys relative to the hydrogen storage alloys of the '591 or '719 patents. A local enrichment of other metals such as Co or a microstructure tuning element itself may also occur.
Nickel metal hydride batteries are replacing nickel cadmium batteries in a large number of applications, owing to environmental concerns and their generally improved performance characteristics. It is to be noted that for purposes of this disclosure the terms “batteries” and “cells” will be used interchangeably when referring to one electrochemical cell, although the term “battery” can also refer to a plurality of electrically interconnected cells.
While nickel cadmium batteries are generally inferior to nickel metal hydride batteries in most regards, they do exhibit superior performance characteristics at ultra-low temperatures (typically −30° C. and below). Consequently, a number of attempts have been implemented in the prior art to improve the ultra low-temperature performance of nickel metal hydride batteries. These prior art approaches generally involve the modification of the base alloy with one or more microstructure tuning elements that act to favorably tailor the properties of the supporting matrix to provide a higher concentration of catalytic metallic particles as well as greater accessibility of reactive species to the catalytic metallic particles. The microstructure tuning elements facilitate an accelerated and directed preferential corrosion of the support matrix during activation or operation to provide a more porous and accessible support matrix that also includes locally enriched concentrations of catalytic metallic particles distributed throughout the surface region of the instant hydrogen storage alloys. As the support matrix becomes more porous and less oxidic, the catalytic metallic particles may become at least partially self supporting. The microstructure tuning elements include Cu, Fe, Al, Zn and Sn. In general, the results achieved in the prior art by such approaches were somewhat limited and were primarily restricted to those metal alloys belonging to the AB5 class.
Presently, there is significant interest in utilizing AB2 type alloys in metal hydride battery systems, due to the fact that AB2 type materials, unlike AB5 alloy materials, generally do not incorporate significant amounts of expensive rare earth elements. Furthermore, batteries incorporating AB2 materials utilizing lightweight metals generally exhibit high gravimetric storage capacities. However, the art has not yet found methods or materials for increasing the ultra low-temperature performance of AB2 type alloy materials. Hence, it will be appreciated that there is a need in the art for methods and materials which can (1) improve the low-temperature performance of the general class of ABx type alloy materials; and (2) there is a particular need for methods and materials which can specifically improve the performance of AB2 type metal hydride alloy materials at ultra low-temperatures.
As will be explained hereinbelow, the present invention is directed to ABx type metal hydride alloy materials which include modifier elements therein which operate to increase the surface area and/or catalytic ability of the alloy so as to thereby increase their low-temperature electrochemical performance in rechargeable battery cells. These and other advantages of the invention will be apparent from the drawings, discussion, and description which follow.